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post #1 of 5 (permalink) Old 03-06-03, 04:19 PM Thread Starter
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BA XR6 Tyre Pressures

Here's something to think about! I started thinking and wondering why Ford recommends 210kpa (30psi, normal/unloaded running) front AND rear, when obviously there is more weight over the front axle than the rear.

I reckon the tyre "footprint" should be the same both ends so I came up with this theory and solution.

Standard tyres on my XR6 are 235/45ZR17 Dunlop SP9000's. Max. load of 650 kilo's @ 51psi each.

I went to the local public weigh station and weighed each axle. The Front Axle carried 1,020 kilo's and the rear 840 kilo's. TOTAL 1,860 kilo's (this included me as the driver, full fuel tank and the normal accessories and items I regularly carry in the vehicle!).

Now here's my reasoning - Maximum Pressure of my tyres (51psi) divided by the max. carrying capacity of both tyres (650k X 2=1,300k) = 0.03923. Now all I did was multiply this figure by the weight of each axle.

Therefore, applying this formula I came up with (front) 1,020(k)X 0.03923 = 40.01PSI, and (rear) 840(k)X 0.03923 = 32.9PSI.

I adjusted the tyres accordingly the next day when cold and added 2psi each end for highway running, ie, 42psi front and 35psi rear. The change has been remarkable! Wet weather handling improved out of sight, steering seems sharper and highway fuel economy is 8.7l/100km@95kph on cruise.

WARNING - Do Not take my figures for granted! Check your own tyres carrying capacity at their maximum pressure rating and use your results as a starting point! Tyre pressures must be adjusted accordingly to varying loads placed in or on the vehicle!

I hope this helps. Cheers!
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post #2 of 5 (permalink) Old 03-06-03, 05:40 PM
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you forgot to mention what body type you have. I assume from another post you have a Ute.

Dont mind the spelling, I'm a life member of the DNA
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post #3 of 5 (permalink) Old 03-06-03, 06:13 PM Thread Starter
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Your 100% right! XR6 in Blueprint, Manual, and a heap of extra's :-)
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post #4 of 5 (permalink) Old 03-07-03, 12:30 AM Thread Starter
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It shouldn't matter what the body type, this formula will work with all body types, and loads.

Hope this helps, Cheers!
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post #5 of 5 (permalink) Old 03-07-03, 12:47 AM
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Good work, I'm impressed. Now to find a weighbridge that isn't closed down.
I always thought recommended tyre pressures were a bit sus. I suspect it is for a softer ride, rather than optimum tyre performance.

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